Resting potential
The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. In most cells the resting potential has a negative value, which by convention means that there is excess negative charge inside compared to outside. The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane. How the concentrations of ions and the membrane transport proteins influence the value of the resting potential is outlined below.
Membrane transport proteins
For determination of membrane potentials, the two most important types of membrane ion transport proteins are ion channels and ion pumps. Ion channel proteins create paths across cell membranes through which ions can pass. They have selectivity for certain ions, thus, there are potassium-, chloride-, and sodium-selective ion channels. Different cells and even different parts of one cell (dendrites, cell bodies, nodes of Ranvier) will have different amounts of various ion transport proteins. Typically, the amount of certain potassium channels is most important for control of the resting potential (see below). Some ion pumps such as the Na+/K+ATPase are electrogenic, that is, they produce charge imbalance across the cell membrane and can also contribute to the membrane potential.
Equilibrium potentials
For most animal cells potassium ions (K+) are the most important for the resting potentialTemplate:Ref. Due to the active transport of potassium ions, the concentration of potassium is higher inside cells than outside. Most cells have potassium-selective ion channel proteins that remain open all the time. There will be net movement of positively-charged potassium ions through these potassium channels with a resulting accumulation of excess positive charge outside of the cell. The net movement of positively-charged potassium ions is due to random molecular motion (diffusion) and continues until enough excess positive charge accumulates outside the cell to form a membrane potential which can balance the difference in concentration of potassium between inside and outside the cell. "Balance" means that the electrical force that acts to move the ions tends to increase until it is equal in magnitude but opposite in direction to the tendency for net movement of potassium due to diffusion. This balance point is an "equilibrium potential". Potassium equilibrium potentials of about 70 millivolts (inside negative) are common in neurons.
For typical animal cells, the most important equilibrium potential is the potassium equilibrium potential. This is because for the resting membrane, the membrane is most permeable to potassium ions and not other ions. This is due to the presence of potassium leakage channels that are open at the resting membrane potential. The Nernst equation is used to estimate the equilibrium potential for an ion. As discussed in the preceding paragraph, the key parameter for such an estimate is the ratio of the concentrations of potassium inside the cell and outside the cell. In many cells, the Nernst potential for potassium ions is a good first approximation of the resting potential. A better prediction of the value of the resting potential can be obtained by also taking into account the activity of electrogenic pumps and ion channels that allow for transmembrane movement of other ions such as sodium and chloride ions.
Measuring resting potentials
In some cells, the membrane potential is always changing (such as cardiac pacemaker cells). For such cells there is never any “rest” and the “resting potential” is a theoretical concept. Other cells with little in the way of membrane transport functions that change with time have a resting membrane potential that can be measured by inserting an electrode into the cellTemplate:Ref. Transmembrane potentials can also be measured optically with dyes that change their optical properties according to the membrane potential.
References
- Template:Note An example of an electrophysiological experiment to demonstrate the importance of K+ for the resting potential. The dependence of the resting potential on the extracellular concentration of K+ is shown in Figure 2.6 of Neuroscience, 2nd edition, by Dale Purves, George J. Augustine, David Fitzpatrick, Lawrence C. Katz, Anthony-Samuel LaMantia, James O. McNamara, S. Mark Williams. Sunderland (MA): Sinauer Associates, Inc.; 2001.
- Template:Note An illustrated example of measuring membrane potentials with electrodes is in Figure 2.1 of Neuroscience by Dale Purves, et al (see reference #1, above).
See also
External links
- Neuroscience - online textbook by Purves, et al
- Basic Neurochemistry Molecular, Cellular, and Medical Aspects by Siegel, et al